How can i be more serious?

well. i'm a little immature. okay, maybe 100% immature. i'm playful. i'm 14 but mentally i'm like 6. my friend was saying " hey, i need somebody to talk to on the phone. " i answered " you have me ;D " and she just stared at me and said " you play too much, your immature and you need to act more your age. besides, i need someone more serious. " i said that " i could be serious. " and my other friend laughed at me and said " you? serious? yeah, that's funny, evie. you can't even be serious on a test or at a business meeting, 'cause that's how immature you are. " and then i got upset. i said maybe i should be more serious today at school. so, here i am asking for help.